There are various high molecular weight olefin polymers and copolymers which are suitable in making extrusion-cast films for commercial purposes. Such extrusion-cast films are generally made by extrusion of the molten polymer onto revolving chilled rolls; the general technique is often referred to as "chill roll extrusion" or "cast film extrusion."
There are, basically, two types of olefin polymerization techniques for preparing high molecular weight olefin polymers and copolymers. The oldest commercial technique involves high pressure, high temperature, and the use of a free radical initiator, such as a peroxide; these type polymers are generally known as low density polyethylene (LDPE) and are also known as ICI-type polyethylenes. These LDPE polymers contain branched chains of polymerized monomer units pendant from the main polymer "backbone" and generally have densities in the range of about 0.910-0.935 gms./cc.
The other commercially-used technique involves coordination catalysts of the "Ziegler" type or "Phillips" type and includes variations of the Ziegler type, such as the Natta type. These catalysts may be used at very high pressures, but may also (and generally are) used at very low or intermediate pressures. The products made by these coordination catalysts are generally known as "linear" polymers because of the substantial absence of branched chains of polymerized monomer units pendant from the main polymer "backbone," and they are also generally known as high density polyethylene (HDPE). It is these "linear" polymers to which the present invention pertains. Linear polyethylene (HDPE) ordinarily has a density in the range of 0.941 to 0.965 gms./cc.
In particular, the present invention pertains to "linear" type ethylene polymers wherein ethylene has been polymerized along with minor amounts of alpha, beta-ethylenically unsaturated alkenes having from 3 to 12 carbons per alkene molecule, preferably 4 to 8. The amount of the alkene comonomer is generally sufficient in the same density of the polymer to be substantially in the same density range as LDPE, due to the alkyl sidechains on the polymer molecule, yet the polymer remains in the "linear" classification; they are conveniently referred to as "linear low density polyethylene" (LLDPE). These polymers retain retain much of the strength, crystallinity, and toughness normally found in HDPE homopolymers of ethylene, but the higher alkene comonomers impart high "cling" and "block" characteristics to extrusion-cast films and the high "slip" characteristic inherently found in HDPE is diminished.
Slip and antiblock agents are often added to LDPE film compounds to insure proper film handling characteristics. "Slip," basically defined as the coefficient of sliding friction, may be adjusted by incorporating, e.g., fatty acid amides. Blocking, the tendency of film to stick to itself, can be reduced by adding finely divided inorganic fillers such as silica. HDPE homopolymers with densities of above about 0.940 gm./cc. generally show good slip properties (that is, they slide across each other with ease), and slip agents are not normally required.
U.S. Pat. No. 3,658,980 is a closely-related invention which discloses addition of various additives, such as polyglycols, to low density ethylene polymers and copolymers for the purpose of increasing the cohesion of films prepared from said polymers.
U.S. Pat. No. 2,879,244 pertains to the incorporation of anti-static agents during the polymerization of ethylenic monomers.
U.S. Pat. No. 3,070,462 pertains to the surface treatment of pre-formed films of low density ethylene polymers to impart adherent properties and cling to such polymers.
U.S. Pat. No. 3,108,011 pertains to rendering textile materials antistatic by the use of polyamines containing polyalkylene oxide groups and other compounds.
U.S. Pat. No. 3,121,914 pertains to a mixer-extruder method and apparatus for adding a fluid to a plastic material.
U.S. Pat. No. 3,387,073 pertains to the use of glycerol as a plasticizer to aid in the dynamic extrusion of high molecular weight (very low melt index) high density ethylene polymers (HDPE) made using a Phillips-type or Ziegler-type catalyst.
U.S. Pat. No. 3,499,950 teaches the use of polypropylene glycols as antistatic agents in olefin polymers and copolymers.